The invention is applicable to interference limited cellular radio systems and particularly to a CDMA system. In the CDMA technique the user's narrowband data signal is modulated by a spreading code, which is more wideband than the data signal, to a comparatively wide band. In the methods, bandwidths from 1 to 50 MHz have been used. The spreading code is conventionally formed from a long pseudo-random bit sequence. The bit rate of the spreading code is much higher than that of the data signal. In order to distinguish spreading code bits from data bits and symbols comprising bits and combinations of bits, the spreading code bits are called chips. Each user data symbol is multiplied by the spreading code chips. Then the narrowband data signal spreads to the frequency band used by the spreading code. Each user has his/her own spreading code. Several users transmit simultaneously on the same frequency band and the data signals are distinguished from one another in the receivers on the basis of a pseudo-random spreading code.
The capacity of interference limited multiple access systems such as the CDMA cellular radio system is determined by an interference power caused by users. In such a system the subscriber terminal usually establishes a connection with the base station to which the path loss is the smallest. The base station coverage does not in all situations correspond to the traffic need, but the load of some base stations increases to such an extent that the connections to the subscriber terminals can be disconnected either due to the increased interference or to the inadequacy of the transmission capacity.
It is assumed in prior art handover and power regulation algorithms that a connection is established with the base station to which the path loss is the smallest. Such a best connection principle is thus preferable, as the traffic load towards the base station is constant or when the signal-interference ratio of the most loaded base station meets the minimum requirement. But when the load of a base station increases to such an extent that the minimum requirements of the connection quality cannot be met, a way is needed to balance the load. A prior art radio system does not, however, allow dynamic load goal management balancing the load, but prior art systems easily lead to an unstable situation, in which disconnecting the connection to some subscriber terminals is the only possibility. Such heavy load situations, in which the connection quality declines below the minimum requirements, and which can thus be called overload situations, are not desired.
In the interference limited radio systems it is of primary importance to keep the load sufficiently low, because otherwise owing to fast power regulation the transmitters increase their power to the maximum. At worst this, in turn, could lead to the disconnecting of most radio system connections. Then again, it is appropriate to simultaneously handle as many connections as possible.
Publication WO 93/09626 shows a method to compensate for the overload. Here a power level is compared with a threshold level. If the received power exceeds the threshold level, the signal interference level of the system is reduced by decreasing the transmission power of the subscriber terminals to correspond to the threshold value. Alternatively the base station determines a pilot signal's signal-to-noise ratio which, in turn, is compared with the threshold value. If the signal-to-noise ratio is lower than the threshold value, the threshold value is reduced and the subscriber terminals are directed to decrease their transmission powers to correspond to a new signal-to-noise ratio. In the solution according to publication WO 93/09626 quality objectives are lowered, when the received total power at the base station increases too much. Here, a drawback is that the solution presupposes that thermal noise can be distinguished from other interference, which is not very easy to implement.